A typical manual powertrain is illustrated in FIG. 1. Solid lines represent mechanical power flow through rotating shafts. Dashed lines represent control connections, which may be implemented using mechanical linkages. Engine 10 generates power at crankshaft 12 by burning fuel. The engine responds to changes in the position of accelerator pedal 14 to generate more power when the pedal is depressed further by the driver. Transmission 16 transmits power from crankshaft 12 to output shaft 18. Transmission 16 includes a friction clutch 20 and a gearbox 22 connected by input shaft 24. Gearbox 22 is capable of establishing a variety of forward speed ratios and at least one reverse speed ratio in response to driver manipulation of shifter 26. The driver controls the torque capacity of clutch 20 by manipulation of clutch pedal 28. Typically, clutch 20 is fully engaged when clutch pedal 28 is released and is fully disengaged when clutch pedal 28 is fully depressed. Differential 30 splits power from output shaft 18 between a left axle 32 driving a left wheel 34 and a right axle 36 driving a right wheel 38 while permitting slight speed differences between the axles as the vehicle turns a corner. In a typical rear wheel drive powertrain, the transmission output shaft is a driveshaft that extends to the differential. In a typical front wheel drive powertrain, the output shaft 18 may be driveably connected to the differential by a final drive gear. The transmission and differential of a front wheel drive powertrain are frequently combined into a single housing and called a transaxle.
For internal combustion engine 10 to generate power, crankshaft 12 must rotate at sufficient speed. However, when the vehicle is stationary with gearbox 22 establishing a speed ratio, input shaft 24 is also stationary. In order to start the vehicle moving, the driver controls the torque capacity of clutch 20 to transmit power from moving crankshaft 12 to stationary input shaft 24. As the vehicle accelerates the speed of input shaft 24 gradually increases until it is equal to the speed of crankshaft 12, at which point clutch 20 can be fully engaged. With clutch 20 fully engaged, the speed of crankshaft 12 is proportional to vehicle speed. As the vehicle accelerates in 1st gear, the speed of crankshaft 12 becomes excessive, necessitating a shift to 2nd gear. Gearbox 22 is not capable of changing ratios while transmitting power. Therefore, the driver shifts by disengaging clutch 20, then manipulating shifter 26 to change the gearbox ratio, then re-engaging clutch 20. Re-engagement of clutch 20 forces the crankshaft speed to become equal to input shaft speed, predominantly by changing the speed of the crankshaft.
There are times when a vehicle is moving but does not require propulsive power. For example, the vehicle may be descending a hill or may be approaching a stop sign. Even though the vehicle does not require propulsive power, the engine generally continues to use fuel and may impose negative torque tending to slow the vehicle down. Driver actions strongly influence the fuel usage and negative torque. The driver may simply release the accelerator pedal and leave the gearbox engaged in the current gear ratio with the clutch fully engaged. In this case, the engine speed will be dictated by the vehicle speed and the selected gear ratio. This causes negative torque as vehicle inertia is used to maintain the engine at this speed. Consequently, the vehicle may slow down prematurely. To compensate for slowing down prematurely, the driver may need to use fuel to re-accelerate, reducing cycle fuel economy. Alternatively, the driver may disengage the clutch and allow the engine to idle. In this state, the negative torque is minimal, but fuel is still used to maintain the engine speed at idle. If the shifts the gearbox to neutral, he may re-engage the clutch without substantially changing vehicle behavior. It is uncommon for a driver to shut the engine off while coasting. Many drivers do not take the actions that would reduce fuel consumption either due to lack of knowledge or due to the effort required.